The Yankee Air Museum evidently put out an email today (Saturday, April 15) to, presumably, ticket holders for those wanting to ride on B-17G 44-85829, other wise known as Yankee Lady, that states the airplane has been grounded by the museum. The text of the email, as copied from this Facebook posting, states, in part:
“The Yankee Air Museum decided to proactively cease flight operations of the B-17G Flying Fortress ‘Yankee Lady.’Recent inspections of other B-17s have discovered wing spar issues. As a result we expect a mandatory Airworthiness Directive to be issued by the FAA in the next few weeks regarding the matter. Out of an abundance of caution, we are temporarily ceasing our B-17 flight operations and awaiting direction from the FAA regarding necessary inspections and repairs that will be required. It is expected that the B-17 will not fly during the 2023 flying season. Please note that this only affects the B-17.“
As one of only three current U.S. operators airworthy B-17s (the others being Ye Olde Pub and Sentimental Journey), the Yankee Air Museum is pretty sure that flyable B-17s will soon be grounded pending another mandatory FAA-required wing inspection. It is a short presumption that this new wrinkle, pun intended, is the result of what has been found on the EAA’s B-17G, 44-85740 (Aluminum Overcast), wing attach structure that caused it to be grounded in April 2021. Whereas the 2001 FAA Airworthiness Directive required inspections of the joint (holes and bolts) between the wing attach tubes and the wing spars themselves, the EAA B-17s issue apparently was related to wing attach structure in the fuselage. I have not been able to find out any detailed information beyond what was posted early by those who should (might) know. From that information, the problem was on the aft wing attach point, fuselage side, at the middle solitary bolt (item “D” in the drawing below). This diagram shows the wing attach structure:
Released information from the EAA indicates that the problem was identified on a fuselage-side shear terminal, one of three attach points for each of the two wing spars on each B-17 wing. Since then, the EAA decided to replace all the fuselage wing-attach fittings for both wings. It is probable that what was discovered on the EAA B-17 is the source of the anticipated FAA AD being issued. There is much (at this point, unfounded) speculation that this new AD will require spar replacements. If the source of the AD is what the EAA is dealing with, it has nothing to do with the wing spars. However, it is possible that the FAA will require a wider inspection and repair of the whole structure…the wing spars, the attach fittings, and the fuselage carry through structure. Without the text of the possible AD available, one can only speculate as to what this possible AD will entail.
The primary structure of the B-17 inner wing panel consists of two truss-type wing spars, forward and rear. Each truss-type spar is built up from two spar chords, upper and lower, that are assembled with tubular members forming “N” type trusses. The upper and lower spars, both front and rear, are themselves attached to the fuselage center section with steel terminal fittings that are bolted into the inner end of the spar chords. The terminal fittings attach to matching wing attach fittings on the fuselage center section that sandwich the wing fittings. The two parts are secured with special tapered pins. Thus, there are four main attach points that hold each wing to the fuselage, along with two other specialized secondary attach points located between the upper and lower spars. The secondary attach points are where the sheer terminal is located.
The spar chords were originally manufactured as an extrusion and many smart people have since tried to figure out how exactly this was done back in the 1940s. The challenging feature of the spar chords is that the inside the hollow square chord tube tapers down from the wing root to the length of the section.
The process to extrude these parts with the exact dimensions as required to match the Boeing design is very specialized and thus expensive. One supplier has been identified but it has proven cost prohibitive to obtain the extrusions for new spar chords. The availability of original wing inner sections is just about zero, so the choice is left to use existing spar chords, manufacture new ones, or re-engineer a refined design for the spar chord. However, trying to re-engineer a new wing spar chord using modern components would require a massive effort and re-certification with the FAA and is not practical.
The other problem with the B-17 inner wing is the steel terminal fittings where each of the four spar chords connect to the fuselage center section. Each of the four attachment fittings are slid into the respective spar chords and precisely held in position with eight close-tolerance bolts. It was determined back in 2001 that the spar chords and the steel attachment fittings were subject to cracking around the holes drilled to hold the bolts. This generated the Airworthiness Directive issued by the FAA in 2001 that requires close inspection of these components to find and repair any cracking found. The AD offers several methods of compliance for the inspection, but if cracking is found then repairs are obviously required. The procedure used to repair the cracked components has to be coordinated with and approved by the FAA.
And now a possible new problem in the B-17 wing attach assembly. If the FAA issues an Emergency AD, it will most likely ground the two remaining flying U.S. B-17s until the AD is accomplished. A new AD would probably affect the UK based Sally-B. Also, it would affect the several other B-17s currently in maintenance or restoration back to airworthy condition. If the FAA targets wide on the wing spars and fuselage structure, it might end up being a showstopper for operators. It should be noted that these 78-year-old airframes were never intended to last more than a few months or years at the most. But, if there are issues with the basic structural integrity of the wings, they bear close scrutiny to ensure that these airworthy B-17s remain safe to fly.
